System and method for producing polyhydroxycarboxylic acid

ABSTRACT

An object of the present invention is to provide a method and a system by which hydroxycarboxylic acid cyclic dimers are efficiently generated, allowing high yields of high-quality polyhydroxycarboxylic acid to be obtained. The method for synthesizing polyhydroxycarboxylic acid comprises a depolymerization step for depolymerizing hydroxycarboxylic acid oligomers to produce hydroxycarboxylic acid cyclic dimers, wherein, in the depolymerization step, a reaction solution is heated via heat transfer from a heat medium passage under reduced pressure while the reaction solution is being flowing through a horizontally provided reaction solution passage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and a method for producingpolyhydroxycarboxylic acid, and a system for producing hydroxycarboxylicacid cyclic dimers.

2. Background Art

Polyhydroxycarboxylic acid is an aliphatic polyester produced bypolymerizing hydroxycarboxylic acid. A typical example thereof ispolylactide.

Examples of known methods for synthesizing polylactide include methodscomprising a step of concentrating lactic acid as a raw material so asto reduce the water content (concentration step), a step of condensinglactic acid to generate oligomers (condensation step), a step ofdepolymerizing such oligomers to generate cyclic dimers(depolymerization step), and a step of subjecting cyclic dimers toring-opening polymerization (ring-opening polymerization step).Polyhydroxycarboxylic acid other than polylactide can also be producedby a technique similar to the above method.

In the concentration step, water contained in the raw material isremoved in order to facilitate the initiation of esterification betweenlactic acid molecules. In the condensation step, oligomers are generatedduring the removal of water resulting from esterification under heatingand depressurization.

In the depolymerization step, in the presence of a depolymerizationcatalyst such as tin octylate as well as under heating and reducedpressure, oligomers are depolymerized and thus lactides, which arecyclic dimer esters of lactic acid, are generated. Lactides aregenerally in gaseous form under the environment of the depolymerizationstep. Hence, lactides are cooled, condensed, and then recovered. Thethus obtained lactides are purified if necessary and then transferred tothe ring-opening polymerization step.

A tank-type reaction container is often used in the depolymerizationstep. WO 93/15127 discloses a method for continuously producing lactidesand lactide polymers, illustrating a tank-type container as a reactioncontainer for generation of lactides.

SUMMARY OF THE INVENTION

When a conventional tank-type reaction container is used in thedepolymerization step, problems occur such that the viscosity of areaction solution increases with time and the lactide yield decreases.Moreover, it is problematic that the optical purity of lactides tends todecrease for reasons such that pressure to be applied to the reactionsolution increases as an increase in the liquid depth, for example. Thelowered optical purity of lactides must be avoided as far as possible,since it results in lowered quality of polylactide as the final product.Therefore, an object of the present invention is to provide a method anda system by which high yields of high-quality polyhydroxycarboxylic acidcan be obtained by efficiently generating hydroxycarboxylic acid cyclicdimers such as lactides.

The present inventors have discovered that hydroxycarboxylic acid cyclicdimers such as lactides can be efficiently generated while preventingoptical purity from decreasing through a depolymerization step by whichdepolymerization is performed while causing a reaction solution to flowhorizontally. The present invention is as summarized below.

(1) A method for synthesizing polyhydroxycarboxylic acid comprising adepolymerization step for depolymerizing hydroxycarboxylic acidoligomers to produce hydroxycarboxylic acid cyclic dimers, wherein,in the above depolymerization step, a reaction solution is heated byheat transfer from a heat medium passage under reduced pressure whilethe reaction solution is being flowing through a horizontally providedreaction solution passage.(2) The method for synthesizing polyhydroxycarboxylic acid according to(1), wherein, in the above depolymerization step, the remaining solutionthat is discharged from an outlet of the reaction solution passage isrecovered in a catch pot and at least a portion of the solution isrefluxed to the reaction solution passage.(3) A polyhydroxycarboxylic acid synthesis system comprising adepolymerization apparatus for depolymerizing hydroxycarboxylic acidoligomers to produce hydroxycarboxylic acid cyclic dimers, whereinthe above depolymerization apparatus has a horizontally providedreaction solution passage, a heat medium passage that is in contact withthe reaction solution passage, and a depressurization apparatus fordepressurizing the reaction solution passage.(4) The polyhydroxycarboxylic acid synthesis system according to (3),wherein, in the above depolymerization apparatus, a plurality ofreaction solution passages are provided in parallel, and the heat mediumpassage is provided so as to enclose the individual reaction solutionpassages.(5) The polyhydroxycarboxylic acid synthesis system according to (3),wherein, in the above depolymerization apparatus, a plurality of heatmedium passages are provided within the reaction solution passage.(6) The polyhydroxycarboxylic acid synthesis system according to any oneof (3) to (5), wherein, in the above depolymerization apparatus, thereaction solution passage and the heat medium passages are provided inparallel.(7) The polyhydroxycarboxylic acid synthesis system according to any oneof (3) to (6), wherein, in the above depolymerization apparatus, a catchpot equipped with a liquid-level meter is provided at an outlet of thereaction solution passage.(8) The polyhydroxycarboxylic acid synthesis method according to (1) or(2), wherein the system according to any one of (3) to (7) is used.(9) A system for producing hydroxycarboxylic acid cyclic dimers, havinga horizontally provided reaction solution passage, a heat medium passagethat is in contact with the reaction solution passage, and adepressurization apparatus for depressurizing the reaction solutionpassage.(10) The system for producing hydroxycarboxylic acid cyclic dimersaccording to (9), wherein a plurality of reaction solution passages areprovided in parallel, and a heat medium passage is provided so as toenclose the individual reaction solution passages.(11) The system for producing hydroxycarboxylic acid cyclic dimersaccording to (9), wherein a plurality of the heat medium passages areprovided within the reaction solution passage.(12) The system for producing hydroxycarboxylic acid cyclic dimeraccording to any one of (9) to (11), wherein the reaction solutionpassages and the heat medium passages are provided in parallel.(13) The system for producing hydroxycarboxylic acid cyclic dimersaccording to any one of (9) to (12), wherein a catch pot equipped with aliquid-level meter is provided at an outlet of the reaction solutionpassage.

According to the system and the method of the present invention, nopressure is applied to a reaction solution since the reaction solutionis depolymerized while the reaction solution is being flowinghorizontally. Therefore, it becomes possible to efficiently generatehydroxycarboxylic acid cyclic dimers such as lactides and to obtain highyields of high-quality polyhydroxycarboxylic acid. Also, according tothe method and the system of the present invention, reduction of thetime for depolymerization reaction, a downsized depolymerizationapparatus, and the like can be realized. The method and the system ofthe present invention are particularly appropriate for continuouslyperforming a depolymerization step.

This specification incorporates the content of the specification ofJapanese Patent Application No. 2010-247481, for which priority isclaimed to the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the system for producingpolyhydroxycarboxylic acid according to the present invention.

FIG. 2 shows a depolymerization reactor, a catch pot, and peripheralapparatuses thereof.

FIG. 3 shows examples of the disposition of reaction solution passagesand heat medium passages in depolymerization reactors.

EXPLANATION OF REFERENCE NUMERALS

1: Lactic acid feed apparatus, 2: Solution transfer pump, 3: Lactic acidconcentration apparatus, 4: Solution transfer pump, 5: Concentratedlactic acid buffer tank, 6: Solution transfer pump, 7: Lactic acidcondensation apparatus, 8: Solution transfer pump, 9: Oligomer buffertank, 10: Solution transfer pump, 11: Oligomer feed pipe, 12: Reactionsolution feed pipe, 13: Mixer, 14: Check valve, 15: Depolymerizationreactor, 16: Catch pot, 17: Reflux pipe, 18: Drainage pipe, 19:Distilling column, 20: Distilling column lower level, 21: Solutiontransfer pump, 22: Lactide purification apparatus, 23: Solution transferpump, 24: Ring-opening polymerization apparatus, 25: Refluxer, 26:Cooler, 27: Depressurization apparatus, 28: Refluxer, 29: Cooler, 30:Depressurization apparatus, 31: Condenser, 32: Cooler, 33:Depressurization apparatus, 34: Depolymerization residue buffer tank,35: Decomposition apparatus for decomposing residue resulting fromdepolymerization, 36: Buffer tank, 37: Catalytic separation apparatus,38: Catalytic removal apparatus, 39: Water separation apparatus, 40:Refluxer, 41: Cooler, 42: Water supply apparatus

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of the system for producingpolyhydroxycarboxylic acid of the present invention. For convenience,production of polylactide with the use of the system in FIG. 1 isdescribed herein. However, the system can be used not only forpolylactide production but also for production of otherpolyhydroxycarboxylic acids. Examples of other hydroxycarboxylic acidsinclude glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid,4-hydroxyvaleric acid, 5-hydroxyvaleric acid, and 6-hydroxycaproic acid.When a polyhydroxycarboxylic acid other than polylactide is produced,the terms “lactide,” “lactic acid oligomer,” and “lactic acid” used inthe descriptions below can be read as “hydroxycarboxylic acid cyclicdimer,” “hydroxycarboxylic acid oligomer,” and “hydroxycarboxylic acid,”respectively. In addition, the term “depolymerization (reaction)” usedherein refers to the generation of hydroxycarboxylic acid cyclic dimer(cyclic ester generated by a dehydration reaction for removing two watermolecules from two hydroxycarboxylic acid molecules) fromhydroxycarboxylic acid oligomers. Therefore, the term “depolymerizationapparatus” may also refer to an apparatus for producinghydroxycarboxylic acid cyclic dimers.

In the lactic acid concentration apparatus 3, water contained in lacticacid is evaporated by heating (lactic acid concentration step). Heatingis preferably carried out within a nitrogen gas flow at 120° C. to 150°C. Water and lactic acid in gaseous form are generated from the lacticacid concentration apparatus 3. These gases enter a refluxer 25 and onlylactic acid is refluxed to the lactic acid concentration apparatus 3.The thus obtained concentrated lactic acid is transferred to a lacticacid condensation apparatus 7 via a concentrated lactic acid buffer tank5.

In the lactic acid condensation apparatus 7, a lactic acid condensationreaction is allowed to proceed for lactic acid oligomer generation(lactic acid condensation step). Water generated during the reaction isevaporated. The term “lactic acid oligomer” used in the specificationrefers to a lactic acid oligomer ranging from a lactic acid dimer to alactic acid polymer having a weight average molecular weight ofapproximately 50,000 at maximum. The weight average molecular weight oflactic acid oligomers obtained as a result of the above lactic acidcondensation reaction generally ranges from 150 to 10,000 and preferablyranges from 500 to 5,000. The lactic acid condensation reaction ispreferably carried out at a reduced pressure of 10 ton or less attemperatures ranging from 120° C. to 250° C. If necessary, catalystssuch as organotin-based compounds (e.g., tin lactate, tin tartrate, tindicaprylate, tin dilaurate, tin dipalmitate, tin distearate, tindioleate, tin a-naphthoate, tin β-naphthoate, and tin octylate) orpowdered tin may be used. Water, lactic acid, low-molecular-weightoligomer, and lactides are generated in gaseous form from the lacticacid condensation apparatus 7. These gases enter the refluxer 28 andthen components other than water are refluxed to the lactic acidcondensation apparatus 7. The thus obtained oligomer is transferred to adepolymerization reactor 15 via an oligomer feed pipe 11 and a reactionsolution feed pipe 12. In addition, when a lactic acid oligomer is usedas a raw material, the lactic acid concentration apparatus 3 and thelactic acid condensation apparatus 7 can be omitted.

In a depolymerization reactor 15, in the presence of a catalyst such astin octylate, lactic acid oligomers are depolymerized to generatelactides that are cyclic dimers of lactic acid (depolymerization step).Crude lactides generated in gaseous form are transferred to a distillingcolumn 19 via a catch pot 16. Residues resulting from depolymerizationare stored in the catch pot 16. The depolymerization reactor 15 and thecatch pot 16 are as described in detail later. In addition, thedepolymerization apparatus in the present invention is meant to compriseat least the depolymerization reactor 15.

In a distilling column 19, vapor containing lactides is cooled and thenimpurities such as oligomers are liquefied and removed. Gaseous lactidesfrom which impurities have been removed are transferred to a distillingcolumn lower level 20. Since liquefied impurities contain high-leveloligomers having molecular weights lowered as a result ofdepolymerization, the liquefied impurities are preferably refluxed tothe lactic acid condensation apparatus 7 and reused as raw materials.

In the distilling column lower level 20, gaseous lactides are cooled andcondensed, and then transferred to a lactide purification apparatus 22.Gases containing much water vapor separated from lactides aretransferred to a condenser 31, so that lactides, lactic acid,low-molecular-weight oligomers, and the like are removed from the gasesand then the resultant is refluxed to the distilling column lower level20. Vapor that has not been condensed in the condenser 31 is liquefiedin an impurity cooler 32 and then discarded. Gases that have not beencondensed in the impurity cooler 32 are released to the outside of thesystem via a depressurization apparatus 33.

A desired example of a distilling column is a surface condenser in whichvapor and a refrigerant indirectly come into contact with each other viametal tubes. This is because direct contact of lactides and oligomerswith a water-containing refrigerant results in acid generation due todecomposition. Acid serves as a catalyst to inhibit the progress of aring-opening polymerization reaction. In addition, acid may causecorrosion of materials such as a distilling column. There areexceptional cases in which a refrigerant that is inert in the presenceof lactides and oligomers is used. In such a case, it is necessary tosufficiently dry a refrigerant to reduce the moisture therein.

The lactide purification apparatus 22 is used for separating targetlactides (mainly L-lactide) from the other impurities for purification.Solvent extraction, melt crystallization, or the like can be used as apurification method. For example, such a melt crystallization methodinvolves cooling melted crude lactides for partial crystallization,forming a solid crystalline phase with low amounts of impurities and aliquid phase with high amounts of impurities, separating the crystallinephase from the liquid phase, causing the crystalline phase to undergosweating, and thus removing impurities. Then, lactides discharged fromthe lactide purification apparatus 22 are transferred to a ring-openingpolymerization apparatus 24.

In the ring-opening polymerization apparatus 24, lactides are subjectedto ring-opening polymerization under reduced pressure of about 10 torror less at temperatures ranging from 120° C. to 250° C. (ring-openingpolymerization step). A ring-opening polymerization catalyst and apolymerization initiator are preferably used for ring-openingpolymerization of lactides.

As ring-opening polymerization catalysts, compounds comprising metalsbelonging to Groups IA, IIIA, IVA, IIB, and VA of the periodic table canbe used.

Examples of catalysts comprising metals belonging to Group IA includehydroxides of alkali metals (e.g., sodium hydroxide, potassiumhydroxide, and lithium hydroxide), salts of alkali metals and weak acids(e.g., sodium lactate, sodium acetate, sodium carbonate, sodiumoctylate, sodium stearate, potassium lactate, potassium acetate,potassium carbonate, and potassium octylate), alkoxides of alkali metals(e.g., sodium methoxide, potassium methoxide, sodium ethoxide, andpotassium ethoxide).

Examples of catalysts comprising metals belonging to Group IIIA includealuminium ethoxide, aluminium isopropoxide, alumina, and aluminiumchloride.

Examples of catalysts comprising metals belonging to Group IVA includeorganotin-based compounds (e.g., tin lactate, tin tartrate, tindicaprylate, tin distearate, tin dioleate, tin α-naphthoate, tinβ-naphthoate, and tin octylate), powdered tin, oxidized tin, andhalogenated tin.

Examples of catalysts comprising metals belonging to Group IIB includezinc powder, halogenated zinc, oxidized zinc, and organozinc-basedcompounds.

Examples of catalysts comprising metals belonging to Group IVB includetitanium-based compounds such as tetrapropyl titanate andzirconium-based compounds such as zirconiumisopropoxide.

Among the above, it is preferable to use a tin-based compound such astin octylate or an antimony-based compound such as antimony trioxide. Inaddition, the amount of such a catalyst to be used herein ranges fromapproximately 1 ppm to 2000 ppm, particularly preferably ranges from 5ppm to 1500 ppm, and more particularly preferably ranges from 10 ppm to1000 ppm with respect to lactide.

As polymerization initiators, for example, alcohols such as 1-dodecanolor compounds having other hydroxy groups can be used.

As the ring-opening polymerization apparatus 24, a vertical reactor, ahorizontal reactor, or a tank-type reactor can be used. Two or morereactors may be aligned in series. Examples of an agitating blade thatcan be used include paddle blades, turbine blades, anchor blades,double-motion blades, and helical ribbon blades.

In the decomposition apparatus 35 for decomposing residue resulting fromdepolymerization, the residue resulting from depolymerization ishydrolyzed under pressurization at temperatures ranging from 120° C. to250° C. through addition of steam. For improvement in the mixingproperties of the residue resulting from depolymerization, it isdesirable to maintain the melted state of the residue resulting fromdepolymerization. Hydrolysis is carried out under pressurization using apressure-proof container (e.g., autoclave) as a reaction container, sothat decomposition can take place while the melted state of the residueresulting from depolymerization is maintained. Alternatively, hydrolysiscan also be carried out, following which the residue resulting fromdepolymerization is solidified and then pulverized for powderization. Aresidue containing lactic acid or low-molecular-weight oligomersresulting from hydrolysis is transferred to a catalytic separationapparatus 37 via a buffer tank 36.

In the catalytic separation apparatus 37, a catalyst contained in theresidue is degraded and separated through addition of an alkaline-earthmetal hydroxide such as calcium hydroxide or magnesium hydroxide. Forexample, when tin octylate is used as a catalyst, it is degraded in aninert gas atmosphere generally at temperatures ranging from 0° C. to250° C. and preferably ranging from 5° C. to 130° C. through addition ofcalcium hydroxide. Thus, tin octylate is degraded to oxidized tin. Theoxidized tin is insoluble to water or lactic acid and thus isprecipitated. A residue containing the degraded catalyst is transferredto a catalytic removal apparatus 38.

In the catalytic removal apparatus 38, the degraded catalyst is removedfrom the residue using a filtration apparatus. Subsequently, the residueis transferred to a water separation apparatus 39, water is separatedtherefrom, and then the resultant is subjected to secondary use as afertilizer such as lactate (e.g., calcium lactate) or discarded.

FIG. 2 shows the depolymerization apparatus of the present inventioncomprising a depolymerization reactor 15 and a catch pot 16, as well asthe peripheral apparatuses. First, a catalyst for depolymerization isadded to oligomers that are transferred from the lactic acidcondensation apparatus 7. As catalysts for depolymerization reactions,compounds comprising metals belonging to Group IVA of the periodic tablecan be used, for example. Specific examples of catalysts fordepolymerization reactions include organotin-based compounds (e.g., tinlactate, tin tartrate, tin dicaprylate, tin distearate, tin dioleate,tin α-naphthoate, tin β-naphthoate, tin octylate, and tin2-ethylhexanoate). A catalyst is preferably used herein in an amountranging from 0.01% by weight to 20% by weight, particularly ranging from0.05% by weight to 15% by weight, and particularly preferably rangingfrom 0.1% by weight to 10% by weight with respect to that of anoligomer.

A reaction solution prepared by mixing an oligomer with a catalyst usinga mixer is supplied to the depolymerization reactor 15. Thedepolymerization reactor 15 has a horizontally provided reactionsolution passage and a heat medium passage that is in contact with thereaction solution passage. The reaction solution passage is connected toa depressurization apparatus 33 (not shown in FIG. 2) via the catch pot16. The reaction solution is heated by heat transfer from the heatmedium passage while the reaction solution is being flowing horizontallythrough the reaction solution passage, and then subjected todepolymerization. The reaction solution is heated in general attemperatures ranging from 120° C. to 250° C. and preferably ranging from120° C. to 200° C. generally under reduced pressure of 100 torr or lessand preferably 10 torr or less.

When excessive pressure is applied to a reaction solution, it isdifficult to maintain a depressurization state, causing a problem suchas inhibition of lactide generation. The reaction solution passage ishorizontally provided, so that no pressure is applied to the reactionsolution unlike a case in which a vertical interval is present in thereaction solution passage. Hence, such a horizontally provided reactionsolution is preferable. In addition, the term “horizontal” in thespecification is meant to include an incline of up to ±1° with respectto the horizontal surface.

The catch pot 16 is provided at an outlet of the reaction solutionpassage. A crude lactide discharge port is provided in the upper part ofthe catch pot 16. The crude lactide discharge port is connected to adistilling column 19. The residue generated by depolymerization isstored in the catch pot 16. A residue discharge port is provided in thelower part of the catch pot 16, and a portion of the discharged residueis circulated to the depolymerization reactor 15 via a reflux pipe.Circulation of such a residue to the depolymerization reactor 15 ispreferable since it leads to an improved lactide yield. A residueportion that has remained uncirculated is discharged to thedecomposition apparatus 35 for decomposing residue resulting fromdepolymerization via a drainage pipe.

A liquid-level meter is preferably provided in the catch pot 16, so thata liquid holdup in the catch pot can be calculated and the residuecirculating amount and the discharging amount can be regulated based onthe result. The fact that the residue circulating amount can beregulated makes it possible to stably operate the depolymerizationreactor 15. In addition, it is preferable to provide flow meters in theoligomer feed pipe, the reaction solution feed pipe, the reflux pipe,and the drainage pipe, since this can facilitate the regulation of theflow rates of the reaction solution and the residue and enables stableoperation of the depolymerization reactor 15. The catch pot 16 may befurther provided with a thermometer.

FIG. 3 (a) to (c) show the examples of the disposition of reactionsolution passages and heat medium passages in the depolymerizationreactor 15. In FIG. 3 (a) to (c), shaded portions correspond to the heatmedium passages and the other white portions correspond to the reactionsolution passages. On the right side in each figure, the cross sectionof a site indicated by A-A′, B-B′, or C-C′ in each figure. In any ofthese embodiments, the reaction solution passages are providedhorizontally.

In the embodiment of FIG. 3 (a), a plurality of reaction solutionpassages having a small inner diameter are provided in parallel within aheat medium passage having a large inner diameter. Specifically, theheat medium passage is provided so that it encloses individual reactionsolution passages. In the embodiment of FIG. 3 (b), a plurality of heatmedium passages having a small inner diameter are provided within areaction solution passage having a large inner diameter. The heat mediumpassages within the reaction solution passage are provided in parallelwith the flow direction of a reaction solution. Also, a heat mediumpassage is also provided in the external part of the reaction solutionpassage. In the embodiment of FIG. 3 (c), a plurality of heat mediumpassages having a small inner diameter are provided within a reactionsolution passage having a large inner diameter. The heat medium passageswithin the reaction solution passage are provided vertical (in adirection perpendicular to the flow direction of the reaction solution)to the flow direction of the reaction solution. Also, heat mediumpassages are provided in the external part of the reaction solutionpassage.

A heat medium passage(s) may be provided either in parallel or verticalto the flow direction of a reaction solution. When pressure to beapplied to a reaction solution is further reduced, a heat mediumpassage(s) is preferably provided in parallel to the flow direction. Onthe other hand, when heat transfer efficiency is increased, a heatmedium passage(s) is preferably provided vertical to the flow directionof a reaction solution. Alternatively, a heat medium passage(s) may beprovided in parallel or at an angle other than vertical to the flowdirection of a reaction solution, as necessary. FIG. 3 (a) to (c) merelyillustrate the embodiments of reaction solution passages and heat mediumpassages in the depolymerization apparatus of the present invention. Thepresent invention is not limited to these embodiments.

In the depolymerization reactor 15 having the above configuration,reaction solution passages and heat medium passages are disposed so thatthey come into contact with each other with a large contact area. Thisenables efficient heat transfer from a heat medium to a reactionsolution and sufficient supply of reaction heat and the heat lost bylactide evaporation. This makes it possible to maintain the temperatureof the reaction solution, leading to improved yield and the stableoperation of the depolymerization apparatus.

The depolymerization apparatus of the present invention is advantageousin that it can be scaled up relatively easily. For example, when aconventional tank-type depolymerization reactor is used, the liquiddepth increases as the apparatus is scaled up, so that pressure to beapplied to the reaction solution also increases. When pressure isapplied to a reaction solution, there are concerns about matters such asdecreased lactide yield and lowered optical purity. However, reactionsolution passages are provided horizontally in the depolymerizationapparatus of the present invention, so that pressure will never appliedexcessively to a reaction solution, causing no such problem. Also, thedepolymerization apparatus of the present invention has relatively asimple configuration, so that a large-scale apparatus thereof can beeasily produced. Furthermore, the depolymerization apparatus of thepresent invention is appropriate for continuously performingdepolymerization.

The present invention also relates to a method for synthesizingpolyhydroxycarboxylic acid. The method for synthesizingpolyhydroxycarboxylic acid of the present invention comprises adepolymerization step for depolymerizing hydroxycarboxylic acidoligomers to produce hydroxycarboxylic acid cyclic dimers, wherein, inthe depolymerization step, a reaction solution is heated by heattransfer from a heat medium passage under reduced pressure while thereaction solution is being flowing through a horizontally providedreaction solution passage. In the depolymerization step of the method, aresidual liquid discharged from an outlet of a reaction solution passageis recovered in a catch pot, and at least a portion thereof is refluxedto the reaction solution passage. This is preferable in view ofimprovement of yield. The method for synthesizing polyhydroxycarboxylicacid of the present invention can be implemented using the above systemfor synthesizing polyhydroxycarboxylic acid of the present invention.

EXAMPLES

The present invention will be explained more specifically with referenceto examples, but the present invention is not limited to the examples.

Example 1

Polylactide was produced using a system for producing polylactide shownin FIG. 1. A lactic acid oligomer having a number average molecularweight of 630 was added as a raw material to a depolymerization reactor15. The embodiment of a reaction solution passage and heat mediumpassages shown in FIG. 3 (b) was employed for the depolymerizationreactor 15. The space within the reaction solution passage wasdepressurized to 10 torr or less at 200° C. Tin 2-ethylhexanoate with aconcentration of 0.7 kg/m³ was used as a catalyst for a depolymerizationreaction. The retention time of a lactic acid oligomer within thedepolymerization reactor 15 was determined to be 15 minutes and theliquid depth within the reaction solution passage was set to be 55 cm.In addition, the lactic acid oligomer retention time was defined bydividing the feed flow-rate of the melted oligomer by the amount(retention amount) of melted oligomer in the depolymerization reactor15, when the feed flow-rate of melted oligomer was equal to the flowrate of condensate obtained from vapor discharged from thedepolymerization reactor 15 and the liquid depth within the reactionsolution passage was stabilized.

A reaction in a ring-opening polymerization apparatus 24 was performedat 200° C. by depressurization to 10 torr or less. The concentration ofthe polymerization initiator was 700 ppm. The weight average molecularweight of the thus obtained polylactide was about 200,000. The yieldbased on raw material was 56%.

Comparative Example 1

A lactic acid oligomer having a number average molecular weight of 630,which was the same as that used in Example 1, was used as a rawmaterial. Polylactide was similarly produced using a conventionalvertical tank-type depolymerization reactor (liquid depth: 5 m) insteadof the depolymerization reactor of the present invention. The weightaverage molecular weight of the thus obtained polylactide was about200,000. The yield based on raw material was 43%.

All references, including any publications, patents or patentapplications cited in this specification are hereby incorporated byreference in their entirely.

1. A method for synthesizing polyhydroxycarboxylic acid comprising adepolymerization step for depolymerizing hydroxycarboxylic acidoligomers to produce hydroxycarboxylic acid cyclic dimers, wherein, inthe depolymerization step, a reaction solution is heated by heattransfer from a heat medium passage under reduced pressure while thereaction solution is being flowing through a horizontally providedreaction solution passage.
 2. The method for synthesizingpolyhydroxycarboxylic acid according to claim 1, wherein, in thedepolymerization step, the remaining solution that is discharged from anoutlet of the reaction solution passage is recovered in a catch pot andat least a portion of the solution is refluxed to the reaction solutionpassage.
 3. A polyhydroxycarboxylic acid synthesis system comprising adepolymerization apparatus for depolymerizing hydroxycarboxylic acidoligomers to produce hydroxycarboxylic acid cyclic dimers, wherein thedepolymerization apparatus has a horizontally provided reaction solutionpassage, a heat medium passage that is in contact with the reactionsolution passage, and a depressurization apparatus for depressurizingthe reaction solution passage.
 4. The polyhydroxycarboxylic acidsynthesis system according to claim 3, wherein, in the depolymerizationapparatus, a plurality of reaction solution passages are provided inparallel, and the heat medium passage is provided so as to enclose theindividual reaction solution passages.
 5. The polyhydroxycarboxylic acidsynthesis system according to claim 3, wherein, in the depolymerizationapparatus, a plurality of heat medium passages are provided within thereaction solution passage.
 6. The polyhydroxycarboxylic acid synthesissystem according to claim 4, wherein, in the depolymerization apparatus,the reaction solution passages and the heat medium passages are providedin parallel.
 7. The polyhydroxycarboxylic acid synthesis systemaccording to claim 5, wherein, in the depolymerization apparatus, thereaction solution passages and the heat medium passages are provided inparallel.
 8. The polyhydroxycarboxylic acid synthesis system accordingto claim 3, wherein, in the depolymerization apparatus, a catch potequipped with a liquid-level meter is provided at an outlet of thereaction solution passage.
 9. The polyhydroxycarboxylic acid synthesismethod according to claim 1, wherein the system according to claim 3 isused.
 10. A system for producing hydroxycarboxylic acid cyclic dimers,having a horizontally provided reaction solution passage, a heat mediumpassage that is in contact with the reaction solution passage, and adepressurization apparatus for depressurizing the reaction solutionpassage.
 11. The system for producing hydroxycarboxylic acid cyclicdimers according to claim 10, wherein a plurality of reaction solutionpassages are provided in parallel, and a heat medium passage is providedso as to enclose the individual reaction solution passages.
 12. Thesystem for producing hydroxycarboxylic acid cyclic dimers according toclaim 10, wherein a plurality of the heat medium passages are providedwithin the reaction solution passage.
 13. The system for producinghydroxycarboxylic acid cyclic dimer according to claim 11, wherein thereaction solution passages and the heat medium passages are provided inparallel.
 14. The system for producing hydroxycarboxylic acid cyclicdimer according to claim 12, wherein the reaction solution passages andthe heat medium passages are provided in parallel.
 15. The system forproducing hydroxycarboxylic acid cyclic dimers according to claim 10,wherein a catch pot equipped with a liquid-level meter is provided at anoutlet of the reaction solution passage.